7 isn't that bad really and i think it happened because of the few battles were you did only average (but it seems it does also count battles where the others don't show up, i need to look into this).
0 = 0.2 %
1 = 1 %
2 = 5.8 %
3 = 14.6 %
4 = 25.3 %
5 = 24.3 %
6 = 15.3 %
7 = 8.2 %
8 = 3.7 %
9 = 1.1 %
10 = 0.4 %
i have 15 battles
14 of them in positive k/d by at list 2:1
have 6 performance.
It looks only at the last 10 battles and you get a score relative to the average of the whole team. So even if you had a k/d of 2:1 there might have been other people doing even better. Or everyone did that good because the other team was just a bunch of peasants with wooden sticks.